If you increased the Doppler transmit frequency during a Doppler exam, which result would be expected?

• A. Penetration would increase
• B. Aliasing would be reduced
• C. Doppler frequency shift would increase
• D. All of the above

Answer: (C)

The key idea is that the Doppler frequency shift scales with the transmitted frequency. The Doppler shift you measure is proportional to the transmit frequency, blood velocity, and the cosine of the angle, given roughly by Δf ≈ (2 f0 v cosθ) / c. So when you raise the transmit frequency, the same moving blood cell velocity and angle produce a larger shift in frequency. That’s why the Doppler frequency shift would increase.

Penetration, on the other hand, depends on frequency-dependent attenuation in tissue. Higher frequency ultrasound attenuates more quickly, so penetration actually decreases, not increases. Aliasing relates to how the Doppler shift compares to the pulse repetition frequency; it isn’t reduced simply by increasing transmit frequency. In fact, increasing the shift without changing the PRF tends to make aliasing more likely. So the only result you’d expect from increasing the Doppler transmit frequency is a larger Doppler frequency shift.